Need some jetting help/ opinions

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dunnroadmafia

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I have a 1978 that i turned into a brat and I need a little input as far as jetting. Ive read the carb guide multiple times and carbs have been cleaned. Mods on the bike consist of uni pods and hughs speedster pipes with mini super trapps mufflers. Carbs are stock BS 38 heres a list of what ive done to them.

Jetted up to 140 mains
Jetted up to 30 pilots
Needle is still at stock 3rd clip position (stock)
Mix screws are 2.5 turns out (stock)
New floats set at 23mm +/- a mm

Carbs have been cleaned well twice and diaphragms looked great no holes or cracking. Problem is that i seem to be running a bit rich. Right now 10 to 20 mph or so the bike gets a bit slugish until the revs really build up. I have 200 miles on a set of NGK BPR5EGP (cross refernce for autolite #63) and they are blacked up a little. That being said should i lean out the needle and drop it to position 4? or do i drop the mains back to the stock 135 size?

Thanks
Dunn
 
Well shit...I don't know how I made that mistake. I've still got the stockers I'll clean them up and reinstall them. But if I'm fouling to hot of plugs I'd imagine my midrange circuit is still to rich right?
 
It might be the float setting. Try 24 mm. 23 mm is a bit high fuel level, this richens the mix.
Leo
 
How many times did you read the Carb Guide? Maybe you better go back and read it again. You seem to have missed the part on needle tuning and clip position numbering .....

NeedleSlots.jpg


Yes, you need to lean your needles, but not by moving them to clip slot 4. That would raise them and make them richer. Put them in clip slot #2.
 
Drop the needles, 5twins? Maybe so, but he's only gone 2 steps up on the mains.

Dunn, spec float level for 78-79 carbs is 24 mm +/- 1 mm, but take note here: as I've posted I don't know how many times before, +/- 1 mm. is NOT A TOLERANCE. It's a tuning range. Go for a tolerance no wider than +/- .005". Also, there's no meaningful "stock" setting for the mix screws once you've changed the pilot jets, intake, and exhaust.

Before you play around with the carbies any more, make sure everything else is right, particularly the ignition. Ignition defects can trick you into thinking the fuel mix is too fat.
 
Yes, drop the needles. The '78-'79 carb set pretty much requires it for ANY increase in main jet size. This is the set where Yamaha increased the main jet size from the previous sets' 122.5 to a 135, a whopping 5 size jump. They had to lean the midrange of course and did so by dropping the needle jet from a Z-8 to a Z-2. Still, the upper portion of the midrange is right on the verge of being too rich right from the factory. It doesn't take much to push it over the edge, and one size up on the mains is usually enough. You may get away with increasing the mains a size or two and not leaning the needles on some of the other 650 carb sets, but not on this one.
 
Thanks for all the input guys. The reason for the floats being set at 23mm is i had read the bowl gasket is a mm thick and if floats are set with the gasket installed you are to compensate for it being there. As for the needle clip slot, i was confused as to the numbering but i have read the carb guide many times and am thankful for the knowledge i did pull from it. I left the mix screws where the cylmer manual called stock because i didnt want to change to much and get confused. This may sound dumb, but this is my first time ever getting into carb work to this extent.

Would the to rich of a needle setting be whats causing that 10 to 20 mph slugish feel or am I barking up the wrong tree? Thanks again for all the help guys.
 
dunnroadmafia said:
Thanks for all the input guys. The reason for the floats being set at 23mm is i had read the bowl gasket is a mm thick and if floats are set with the gasket installed you are to compensate for it being there. As for the needle clip slot, i was confused as to the numbering but i have read the carb guide many times and am thankful for the knowledge i did pull from it. I left the mix screws where the cylmer manual called stock because i didnt want to change to much and get confused. This may sound dumb, but this is my first time ever getting into carb work to this extent.

Would the to rich of a needle setting be whats causing that 10 to 20 mph slugish feel or am I barking up the wrong tree? Thanks again for all the help guys.
Click to expand...

Assuming you still have the mechanical ATU, that 10 to 20 mph sluggish feel can be caused by the ATU springs being too strong. That can happen if someone has played with the springs, such as cutting off one coil, or just old age of the springs. Perhaps even a sticky ATU rod due to poor cleaning/lubrication.

The timing does not advance fast enough, and the retarded timing makes the engine bog and seem sluggish.

That's why the E-advancer on the Pamco is a real nice upgrade. It gives a linear reliable advance curve, meaning linear acceleration.
 
I do have the mechanical atu, it is a repop that came with the pamco kit. The advance rod was liberally greased with moly grease when i installed it. I did check it for movement a couple weeks ago and it open and snaps back fine.
 
I'm telling you, lean those needles. It will make all the difference in the world. When tuning these CV carbs, you consider more the RPM range you're running in than the speed you're going. That RPM range will indicate what circuit you're running in (idle, midrange, main) so will tell you what component needs tinkering with. The needles will effect from just off idle, say 2 to 2.5K, all the way up to 5.5K or so. You also have to realize the carb circuits overlap and somewhat effect the ones next to them .....

Carb_Circuits.jpg


The bleed-over from your larger mains and pilots has made your midrange too rich, all of it, both at its bottom and upper limits.
 
Only reason I asked again was to make sure I completely understood what i had read. I dont want to be looked at as not pay attention or disregarding information here. I will take them up to notch #2 and report back.

Thanks again
Dunn
 

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